The present invention generally relates to the design of orifices used in an inkjet printer printhead and more particularly relates to orifices having at least one axis of asymmetry disposed in the orifice plate of an inkjet printer printhead.
An inkjet printer operates by positioning a medium, such as paper, in conjunction with a printing mechanism, conventionally known as a print cartridge, so that droplets of ink may be deposited in desired locations on the medium to produce text characters or images. The print cartridge may be scanned or reciprocated across the surface of the medium while medium is advanced increment by increment perpendicular to the direction of print cartridge travel. At any given point in the print cartridge travel and medium advancement operation, a command is given to an ink ejection mechanism to expel a tiny droplet of ink from the print cartridge to the medium. If the mechanism of ink expulsion is a thermally induced boiling of ink, the ink expulsion mechanism consists of a large number of electrically energized heater resistors which are preferentially heated in a small firing chamber, thereby resulting in the rapid boiling and expulsion of ink through a small opening, or orifice, toward the medium.
A conventional print cartridge for an inkjet type printer comprises an ink containment device and an ink-expelling apparatus, commonly known as a printhead, which heats and expels the ink droplets in a controlled fashion. Typically, the printhead is a laminate structure including a semiconductor or insulator base, a barrier material structure which is honeycombed with ink flow channels, and an orifice plate which is perforated with circular nozzles or orifices with diameters smaller than a human hair and arranged in a pattern which allows ink droplets to be expelled. Thin film heater resistors are deposited on or near the surface of the base and are usually protected from corrosion and mechanical abrasion by one or more protective layers. The thin film heater resistors are electrically coupled to the printer either directly via metalization on the base and subsequent connectors or via multiplexing circuitry, metalization, and subsequent connectors. Microprocessor circuitry in the printer selectively energizes particular thin film heater resistors to produce the desired pattern of ink droplets necessary to create a text character or a pictorial image. Further details of printer, print cartridge, and printhead construction may be found in the Hewlett-Packard Journal, Vol. 36, No. 5, May 1985, and in the Hewlett-Packard Journal, Vol. 45, No. 1, February 1994.
Ink flows into the firing chambers formed around each heater resistor by the barrier layer and the orifice plate and awaits energization of the heater resistor. When a pulse of electric current is applied to the heater resistor, ink within the firing chamber is rapidly vaporized, forming a bubble which rapidly ejects a mass of ink through the orifice associated with the heater resistor and the surrounding firing chamber. Following ejection of the ink droplet and collapse of the ink bubble, ink refills the firing chamber and forms a meniscus across the orifice. The form and constrictions in channels through which ink flows to refill the firing chamber establish the speed at which ink refills the firing chamber and the dynamics of the ink meniscus.
One of the problems faced by designers of print cartridges is that of maintaining a high quality of result in print while achieving a high rate of printing speed. When a droplet is expelled from an orifice due to the rapid boiling of the ink inside the firing chamber, most of the mass of the ejected ink is concentrated in the droplet which is directed toward the medium. However, a portion of the expelled ink resides in a tail extending from the droplet to the surface opening of the orifice. The velocity of the ink found in the tail is generally less than the velocity of the ink found in the droplet so that at some time during the trajectory of the droplet, the tail is severed from the droplet. Some of the ink in the severed tail rejoins the expelled droplet or remains as a tail and creates rough edges on the printed material. Some of the expelled ink in the tail returns to the printhead, forming puddles on the surface of the orifice plate of the printhead. Some of the ink on the severed tail forms subdroplets (xe2x80x9csprayxe2x80x9d) which spreads randomly in the general area of the ink droplet. This spray often lands on the medium to produce a background of ink haze. To reduce the detrimental results of spray, others have reduced the speed of the printing operation but have suffered a reduction in the number of pages which a printer can print in a given amount of time. The spray problem has also been addressed by optimizing the architecture or geometry of the firing chamber and the associated ink feed conduits. In many instances, however, very fine optimization is negated by variables of the manufacturing process. The present invention overcomes the problem of spray and uncontrolled tail without introducing a reduction in print speed or fine ink channel architecture optimizations.
A printhead for an inkjet printer and methods for making and using the printhead includes an ink ejector and an orifice plate having at least one orifice from which ink is expelled, extending through the orifice from a first surface of the orifice plate abutting the ink ejector to a second surface of the orifice plate. The at least one orifice has at least one axis of symmetry.